1. Field of the Invention
The present invention relates to an optical transmission apparatus, an optical repetition apparatus and an optical transmission method for use in an optical communications system, and in particular to an optical transmission apparatus, an optical repetition apparatus and an optical transmission method in which a signal light is output for which compensation has been made for wavelength dispersion in the transmission path.
2. Description of the Related Art
It is well known that in ultra high speed optical communication systems waveform distortion resulting from the wavelength dispersion characteristics of an optical fiber produces a deterioration in the quality of the transmission. In order to reduce the effect of this wavelength dispersion, optimization of so-called "chirping" is used, where a fluctuation of suitable magnitude and in a suitable direction is generated in the wavelength of the signal light waveform at the rising and falling of the signal. For example, when transmitting a 1.55 .mu.m light signal through a 1.3 .mu.m band zero dispersion fiber, by lengthening the wavelength of the signal light at the rising, and shortening the wavelength at the falling, it is possible to create a signal light waveform which retains good form following transmission through the optical fiber.
The aforementioned chirping is conducted by using a device such as a semiconductor light modulator. It is known that with this type of light modulator the phase modulated component which is generated causes the fluctuation in the wavelength of the light (the chirp). The amount of chirping generated can be expressed in terms of an .alpha. parameter. The .alpha. parameter is a quantity determined by the operating wavelength and the absorption spectra of the light modulator, and the degree of variation in the dispersion of the refractive index. Thus, chirping can be created in the signal light by considering the .alpha. parameter and then setting the operating conditions of the light modulator accordingly.
In current optical transmission and optical repetition apparatus, compensation for wavelength dispersion within an optical fiber has been made by using a light modulator as described above to cause a chirp in the signal light which is of an appropriate magnitude for the wavelength dispersion characteristics of the connected optical fiber.
However, the polarity of the appropriate chirp to be created in the signal light will vary depending on the type of optical fiber through which the signal light is to be transmitted. Chirping generated by a semiconductor light modulator however, is generally of a fixed polarity. Consequently, with current optical transmission and optical repetition apparatus, the altering of the polarity of the chirp in the signal light in accordance with the transmission path being used has been problematic.
Techniques for altering the polarity of a chirp include the use of a nonlinear optical phenomenon known as a four wave mixing effect, and this technique has been reported in Japanese Unexamined Patent Publication No. 63-103215 and Japanese Unexamined Patent Publication No. 7-312574.
The technique outlined in the former of the above two publications involves compression of a light pulse by taking a high output light pulse in which a positive chirp has been generated by transmission through an optical fiber, and following conversion via a four wave mixing medium to a light pulse with a negative chirp, re-transmitting the pulse through the optical fiber. The object of this technique then is to achieve compression of the light pulse by utilizing the wavelength dispersion characteristics of the optical fiber, and it is not possible to generate a signal light in which compensation has been made for the wavelength dispersion characteristics of the optical fiber, as is the object of the present invention.
The technique outlined in the latter of the above two publications involves the connection of two optical fibers with equal amounts of wavelength dispersion to either end of a phase conjugated light generating apparatus which utilizes a four wave mixing effect, so that the wavelength dispersion generated in each of the optical fibers is cancelled out. However with this technique, if it becomes necessary to replace one of the optical fibers connected to the phase conjugated light generating apparatus, then in order to maintain equality in the amount of wavelength dispersion before and after the phase conjugated light generating apparatus it is necessary to replace both of the optical fiber. Consequently, dealing with modifications in an optical communication system using this technique is problematic.